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April 20, once a counterculture holiday when potheads would gather to celebrate and smoke pot, is turning into a blockbuster event that could potentially bring in millions in tax and tourism dollars for states where it's newly legal.

Denver is playing host to its fifth annual Cannabis Cup this weekend, and this year's event is expected to be city's the largest yet in the wake of legalization.

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An estimated 30,000 visitors were expected at the two-day event, according to The New York Times. At the start of the month, event organizers had already sold about 10,000 more tickets than they had last year, when pot wasn't yet legal in the state except for medical use.

Searches for hotel rooms in Denver on Hotels.com for the 4/20 weekend were up 73 percent over last year, too, according to The Cannabist -- a spike the company attributed in part to the state's legal recreational pot sales.

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Scroll down to read some facts and figures about marijuana legalization and usage in the U.S.

4/20's hazy history

While there are no shortages of theories about how the “high" holiday came to be, several published reports give the credit for 4/20's creation to a group of Northern California high school students. The friends say they started using the term as code for pot-smoking in 1971, after planning to meet at 4:20 p.m. one day to smoke and search out a rumored pot crop.

The term spread, eventually reaching, through mutual acquaintances, members of the Greatful Dead rock band, the friends claim. The lingo was picked up by High Times magazine in 1990, according to BBC News, after an editor saw the term on a Grateful Dead concert flyer.

Marijuana is the most commonly used illicit drug in the country, a 2012 government survey found. An estimated 18.9 million Americans aged 12 and older -- or 7.3 percent of them -- said they had used marijuana in the past month.

And the numbers of Americans who say they have tried pot are far higher. A Pew Research survey taken in February found that nearly half of Americans polled, or 47 percent of them, admitted to having used marijuana at some point in their lives.

Medical research ramps up

The casual use such numbers suggest was the target of a study, published this week, that analyzed the effects of recreational pot-smoking on two neural regions that regulate emotions and motivation.

That study, a collaboration between researchers from Harvard's and Northwestern's medical schools, found the size, shape and density of both the amygdala and the nucleus accumbens significantly altered, even in people smoking as little as once a week. Those who smoked pot more regularly had abnormally large nucleus accumbens.

"When we saw that there was a consistent abnormality and that it was directly related to the amount of cannabis one took in, it gave us some significant pause," the study's co-author said. "Seeing these differences raises a cautionary flag that we need to do more research."

Seeing green

That need for more medical research hasn't stanched the interest in pot's profit potential.

Pot is already doing a brisk business in Colorado alone, and the taxes could provide a windfall for the state. A Moody's report predicted that the state will add an estimated $98 million to its tax coffers this year from taxes on recreational pot use on Jan. 1.

One group of industry investors seeking to capitalize on legal pot estimated in a recent report that the current market value is about $1.53 billion, a figure that includes all states that have active sales to those who are allowed to possess marijuana under state law.

That group, ArcView Market Research, also projected that Washington and Colorado would add $253 million and $455 million respectively their markets in 2014.

As for other states? The national pot market has a potential to swell to $10.2 billion in five years, according to the report. (To provide some perspective, the world's best-selling drug Lipitor at one point peaked at $13 billion in annual profit.)

Sweeping changes

Americans' attitudes toward marijuana are evolving, too, with a recent Pew study finding that a majority of Americans — 54 percent — now think pot should be legal.

Voters in both Colorado and Washington signed off in 2012 on steps to legalize marijuana for personal recreational use, while other states rethink their own laws on a smaller scale.

Forty states have eased their drug laws since 2009, a recent Pew Research Center analysis found. In addition to Colorado and Washington, another 15 states and the District of Columbia have decriminalized certain amounts of possession, while even more have legalized it for medicinal use.

The Justice Department appears to be giving such efforts the green light. Attorney General Eric Holder has given Washington and Colorado the go-ahead for their state legalization initiatives, as well as called for major changes to federal drug sentencing guidelines.

Those signals from the feds, coupled with polls suggesting more Americans support legal pot, may mean more states could be poised to head in a similar direction.